The present invention related to servo control systems which may be used in controlling the head positioning actuators in storage devices such as disk drives. More specifically, the invention relates to digital servo control systems involving efficient implementations of tracking and seeking servo control methods.
Various servo control systems are known in the art. U.S. Pat. No. 3,458,785 (Sordello) discloses an early example of a servo control system employing quadrature signals for providing position and velocity information, the system having fine and coarse positioning control algorithms. U.S. Pat. No. 4,135,217 (Jacques et al.) discloses a system which compensates for repeatable errors such as wobble of a disk, with stored xe2x80x9crun-outxe2x80x9d information being used to compensate for the errors. U.S. Pat. No. 4,412,161 (Cornaby) generally discloses a digitally implemented recursive servo control system. U.S. Pat. No. 4,486,797 (Workman) discloses a servo control system in which a preprogrammed velocity profile is used. U.S. Pat. No. 4,783,705 (Moon et al.) discloses an embedded sector servo system divided into separate track and seek control systems, the system including an automatic gain control function. U.S. Pat. No. 4,788,608 (Tsujisawa) discloses a control system for positioning read/write heads over a non-circular track, by using harmonics of the rotation period of the disk. U.S. Pat. No. 4,835,632 (Shih et al.) discloses a servo control system having different sampling and control frequencies for tracking and seek operations, the system apparently scaling or normalizing certain values during operation. U.S. Pat. No. 4,835,633 (Edel et al.) discloses a servo control system involving values calibrated for voice coil motor acceleration as a function of radial position using a polynomial curve fit.
U.S. Pat. No. 4,879,612 (Freeze et al.) discloses a servo control system claiming a purely digital seek mode but a hybrid tracking mode, the system using track identity information and sample testing to reduce errors. U.S. Pat. No. 4,890,172 (Watt et al.) discloses an automatic gain calibration system for a disk drive servo system. U.S. Pat. No. 4,907,107 (Sakurai) discloses a low level circuit for generating signals corresponding to sampled magnetic pulses. U.S. Pat. No. 4,914,644 (Chen et al.) discloses a servo control system including a model of coil current involving comparison of a velocity error to a predetermined value during long seeks, when the power amplifier is saturated. U.S. Pat. No. 4,914,725 (Belser et al.) discloses a servo control system including a xe2x80x9cpiggy-backxe2x80x9d construction including a fine positioner carried by a coarse positioner, the dynamic range of the fine positioner being increased momentarily at the time of track capture, this patent naming a common inventor with an inventor of the present patent application. U.S. Pat. No. 4,942,564 (Hofer et al.) discloses a gain compensation system in which a test signal is introduced to determine the system""s response for comparison to a previously stored value. U.S. Pat. No. 4,954,909 (Sengoku) discloses a system for determining movement of a recording head, especially for determining when it has hit a disk surface. U.S. Pat. No. 5,038,333 (Chow et al.) discloses a track-seeking apparatus having a track crossing detector for providing position information, this patent naming an inventor who is also a named inventor in the present patent application.
European Patent Application 0,390,467 (Ogino) discloses a digital servo control system in which certain variables are not calculated when an error value is substantially zero. Japanese Patent Document 63-316380 discloses a control system having a plurality of servo bit sample periods.
These patents, as well as any documents cited in this patent application, are incorporated herein by reference in their entirety.
In the field of magnetic disk drives, servo systems are required to accurately position read/write heads over a given substantially circular track on the disk, as well as efficiently move the heads from above one track to a new desired position above a second track. It is desirable in the xe2x80x9ctrackingxe2x80x9d (or xe2x80x9cpositionxe2x80x9d) mode that the position of the read/write heads with respect to the track be maintained in the proper center position above the track. Similarly, when moving the heads from one track to another in the xe2x80x9cseekxe2x80x9d (or xe2x80x9cvelocityxe2x80x9d) mode, it is desirable that the heads become stably centered above the destination track as soon as possible. These goals must be met even in the presence of anomalies such as changes to or occurrences in the disk drive mechanism, or deviations from perfect circularity in the tracks. These anomalies may involve aging, temperature changes, changes in orientation of the disk drive, humidity, shock and vibration. Specific performance objectives embodying these broad goals include reduction of tracking error (expressed as a percentage of the radial separation of the tracks), average access time (reflecting the average time required to move the read heads to a destination track in a typical read operation and provide the outside world with data from the disk), and bit error rate (BER, in bits per 10n).
Meeting these goals allows the disk drive""s performance to be improved. If these goals are not met, misalignment or delayed alignment of the read/write heads with respect to the tracks cause increases in read/write errors and a slowdown in read or write operations.
Further objectives include reduction of the size of the disk drive itself. As recording densities improve, a hindering factor in reducing overall disk drive physical dimensions may be the size of the circuitry required to implement the servo system. Therefore, there is a need to provide a high-performance, reliable, and fast servo control system which is both economic and compact.
More specifically, various schemes are known today for placing position information on the surface of disks so that the position of the head over the disk can be determined. This position information, commonly referred to as servo information, may in some schemes occupy an entire surface of one disk. This xe2x80x9cdedicatedxe2x80x9d scheme has the disadvantage that it occupies a substantial portion of the total area allotted for useful information. For example, in a two-disk system, there are four surfaces. If one of the surfaces is dedicated to servo information, at least 25% of the otherwise useable surface of the disks is used, purely for positioning information. As the physical size of disks becomes smaller with the progress of technology, dedication of an entire disk surface to servo information becomes increasingly unacceptable.
In a second scheme, called an xe2x80x9cembeddedxe2x80x9d servo design, the servo information is recorded on every disk surface along with the user data areas. Although embedded servo designs have increased the servo area efficiency over dedicated schemes, known embedded servo schemes have typically involved complex servo data fields, which has forced larger amounts of disk area to be allocated to the positioning information. As the amount of area dedicated to positioning information increases, either the amount of useable data decreases, or the density of useable data storage increases, both of which are undesirable. Therefore, there is a need in the art to provide a servo control system in which there is no surface dedicated entirely to servo control information, and in which the area allocated to servo control information is reduced to a minimum while maintaining optimum seeking and tracking efficiency.
Known servo systems involve analog circuitry. Use of analog circuitry in servo systems can involve reduced tolerance to noise. Clearly, noise-contaminated signals cause degradation in system performance, so that a commensurate reduction in noise-intolerant components is desirable. Therefore, there is a need in the art to provide a totally digitally-implemented servo control system.
The disadvantages of analog or hybrid circuits are not limited to noise intolerance. Analog or hybrid circuits have typically been larger in size than purely digital circuits. Furthermore, at least partially due to the attempt to partially overcome noise-related problems, analog or hybrid servo control systems have required more than one power supply to be present. Therefore, design objectives such as miniaturization and reduction in the number and output requirements of power supplies, further point out a need for a totally digitally-implemented servo control system.
There are known control systems which are partially implemented using digital hardware. However, many of these systems have several parameters which are frozen at the time of system design, so that the parameters must be selected to be the most acceptable compromise for all operations. For example, the parameters in fixed-parameter systems must be chosen so as to function during both seeking operations and during tracking operations. This compromise degrades performance in each operation, as compared to a system optimized for a tracking operation and optimized separately for a seeking operation. Therefore, there is a need in the art to provide a servo system in which servo parameters may be adaptively changed in accordance with the operation currently being executed by the servo controller.
On a matter related to operation-specific parameter optimization, it is known that age, temperature, humidity, and other environmental factors cause deterioration in system performance. Freezing system parameters at the time of design limits the disk drive""s long-term performance under these changing conditions. Therefore, there is a need in the art to provide a servo system in which parameters may be adaptively calibrated over time, as these environmental changes occur.
The present invention provides a highly adaptive, responsive, comprehensive servo control system allowing stable tracking and efficient seeking in the presence of a variety of structural anomalies and adverse occurrences such as temperature variation, change in physical orientation, shock, vibration, and humidity.
The invention provides adaptive compensation for a variety of tracking and seek problems and is ideally implemented in digital form for accuracy and speed. A controlled entity such as actuator coil current is determined by a control effort signal output from a digital signal processor (DSP) or microcontroller.
In a first embodiment, the DSP may operate in at least a tracking mode or a seek mode. Especially in the first embodiment, repeatable runout which is a function of the angular position of the heads with respect to the rotating disks, and bias which is a function of the radial position of the heads, are adaptively compensated. Further, a one-track seek controller is specially provided within the tracking mode for seeking an adjacent track. A DC offset compensator is provided to correct for offsets in an input position error sensor. In the seek mode, a reference velocity deceleration compensation function is performed to optimize seek and access time for the particular disk drive, this function also being adaptively controlled. Both the tracking servo controller and the seek servo controller may be influenced by a bandwidth compensation function. Compensation for this variety of anomalies is preferably performed in a totally digital manner, speeding operation and simplifying modification of the servo controller""s design and parameters.
According to the present invention, especially a second embodiment thereof, an embedded servo design is provided, the servo field occupying a minimal amount of disk surface space. Content of the servo field provides a maximum amount of information to work with a full state-space observer in the servo system. The observer provides predicted state values to allow optimum tracking and seek performance. The servo system is implemented in totally digital form, using firmware to implement a set of functions having dynamically scalable parameters and adaptively calibrated compensation functions. According to the present invention, the digital implementation provides a disk drive with an extremely small form factor, requiring only a single (for example, 5 volt) power supply which consumes a minimal amount of power. Dynamically scaled servo system parameters optimize system performance during successive portions of a seek operation and during close tracking mode. Further, various compensation schemes within the firmware-based servo controller maintain optimum servo performance despite environmental changes and component degradation. Moreover, the compact storage of digital servo information on the disk, and the intelligent prediction models in the servo control system, allow the system to quickly adapt and compensate for unpredictable occurrences, such as vibration and shock, thus making the adaptation and compensation time imperceptible or barely perceptible to the user. In particular, the second embodiment of the present invention allows 120 MB (formatted) storage on four 3.5-inch diameter disk surfaces (two disks), in a package measuring less than 4 inches wide, 1.0 inches high, and 5.75 inches deep.
Other features and advantages of the present invention are apparent from the following Detailed Description of the Preferred Embodiments and the accompanying drawing figures.